The present invention relates to a voltage comparator circuit for comparing two voltages having a small difference in value therebetween.
In a conventional voltage comparator circuit fabricated in a semiconductor integrated circuit of a complementary MOS (CMOS) structure employing both of N-channel transistors and P-channel transistors, a two-stage amplification structure is employed in which a differential amplifier receives a pair of input voltage signals to be compared and an inverting amplifier amplifies the output voltage of the differential amplifier. Although two two-stage amplification structure realizes an amplification factor which is usually 2000 to 5000, it is usually provided with one more inverting amplifier so as to establish margin in the amplification. In such a voltage comparator circuit, it is necessary to increase the number of amplification stages in accordance with a decrease in an input voltage difference. This causes an increase in the area occupied by the amplifiers in the integrated circuit, and also an increase in the power consumption. Moreover, an in-phase voltage may not be perfectly removed by the differential amplifier in the first stage. When an in-phase voltage component of the input voltage varies in the case where an input voltage difference is not more than several mV, an output voltage from the differential amplifier varies, and such an output voltage is then amplified by the inverting amplifier. Consequently, in an output signal from the final stage, the logic "1" state and logic "0" state are erroneously exchanged in some cases when an inphase voltage is at a certain level. The same phenomenon occurs when a power source voltage varies. Therefore, it is impossible for a voltage comparator circuit to determine a voltage difference of not more than several mV.